Galvanized steel material and process for producing same



April 6, 1965 w. c. SIEVERT 3,177,088

GALVANIZED STEEL MATERIAL AND PROCESS FOR PRODUCING SAME 2 Sheets-Sheet 1 Filed April 28, 1961 April 6, 1965 w. c. SIEVERT GALVANIZED STEEL MATERIAL AND PROCESS FOR PRODUCING SAME Filed April 28. 1961 2 Sheets-Sheet 2 MUQ ARQW KWEQ R RN m mm mm DD R n o \\\1 Us a QM QM as QM, Om 0A gray, spangle-free appearance.

United States Patent FGR PRODUCING SAME William C. Sievert, Chesterton, Ind, assignor to Inland Steel Company, Chicago, Ill., a corporation of Delaware Filed Apr. 28, 1961, Ser. No. 106,324

. Claims. ((31. 111-56) The present invention relates generally to galvanized steel material and more particularly to galvanized steel material having a smooth dark gray coating which is readily paintable, which -is highly adherent to the steel base, and which has excellent corrosion-resistant properties; and to a process for producing this material.

Essentially, the subject material is produced by heating galvanized steel material to a temperature at which Fe from the steel base alloys with the Zn in the coating and until the iron content at the outer surface of the coating is at least about 7%.v This alloying step may be carried out immediately after the zinc coating has been applied F 3,177,083 Fatented Apr. 6, 1965 ICC FIGURE 3 is a diagrammatic view of a third embodiment of a process for producing the subject material; and

FIGURE 4 is a graph showing the iron content curves for coatings on various galvanized steel materials.

With reference to the process illustrated diagrammatically in FIGURE 1, a strip of steel material 10 from a coil 11 is initially passed through an oxidizing furnace 12 to produce a thin film of oxide coating on the surface of thevsteel strip. Strip 10 is then passed through a reducing furnace 13 which causes a reduction of the oxide coating on the surface of the steel and the formation of a tightly adherent impurity-free surface layer of metal on and while the zinocoating 'is stillmolten; or it may be carried out at any time after the galvanized material has cooled following coating.

I The coating on the subject material has a smooth, dark The iron content of the coating varies from the outer surface of the coating to the interface between the coating and the steel base and is characterized by a concave curve in the plot of iron content versus distance fromthe outer surface of the coating. The varying iron content and the shape of the iron gradient curve can be determined by X-ray spectroscopic analysis or other methods well known to those skilled in the art. I

Itis essential that the coating have an iron gradient characterized by the concave curve described above. If the curve departs from a concave line the adherence properties of the coating become poorer, eventually resulting in a relatively brittle dark gray coating as the curve approaches aconvex line. It has been discovered that a the material to an additional alloying treatment, said salvaging constituting a feature of the present invention.

The smooth, spangle-free,' adherent, relatively ductile dark gray coating on the subject galvanized material renders this'material ideal forpainting purposes No preliminary processing is required 'to condition this material to receive a coating of paint. It is possible that the dark gray color-of the coating might'be considered objectionable if the galvanized material were to be used in an unpainted condition. However, since the material is intended for painting purposes, the color of the coating is of no particular concern.

the steel strip. The strip 10 is then passed over an idler roller 14 and through a hood 15into a zinc bath '17 contained in a dip pot 16 wherein a coating of zinc is applied to the surface of the steel strip. The steel strip is moved around an idler roller 18 in dip pot 16 and then upwardly through a pair of thickness-regulating rollers 19 which control the thickness of the coating on the strip. Up to and including the thickness-regulating step the process is conventional and is described in greater detail in US. Patent No. 2,110,898, issued March 15, 1938.

In accordance with the present invention, the steel strip coated'with a layer of zinc is then subjected to an alloying treatment by moving the strip through a furnace 20. Performing the alloying step immediately after the coating step capitalizes on the zinc coating. still being in a substantially molten state, thereby facilitating alloying between the iron and the zinc, and accordingly cutting down the length of furnace needed to produce the desired amount of alloying.

Foliowing the alloying step, the galvanized strip is cooled by artificial cooling means 21 such as a water spray chamber, or allowed to cool in air, and then moved over a roller 22 and wound into a coil 23.

The temperature in the alloying furnace 2i) and the length of time for which the galvanized strip 'must be exposed to the alloying temperature vary in accordance with the thickness of the strip and the thickness of the zinc coating thereon. For purposes of illustration, it is assumed that the strip has a thickness of 14' to 24 gauge with a zinc coating of 0.5-1.5 ounces- (preferably 0.7 to

0.9; ounce) of zinc per square foot.

can then be obtained if furnace ztl is maintained at a The dark gray coating of the subject material, also a possesses superior corrosion-resistance properties, as exemplified by an inability to readily strip this coating with a by. weight, HCl stripping solution. The corrosionresistance properties of this coating are better than those of the light gray coating described in US. Letters Patent 7 No. 3,056,694. I

Other features and advantages of the subject invention will become apparent to those skilled in the art from FIGURE 2=is a fragmentarydiagrammatic view illustrating anotherembodiment of a proces' sfor producing the subject material;

temperature of about 1800 F. to. 1900 F. and if the.

concave shaped curve C in the graph of FIGURE 4 which plots Fe content against distance from the outer surface of the coating.

Furnace 26 maybe a conventionally constructed continuous heat treatment furnace of a type well known to those skilled in the art, and may include conventional damper means or the like for admitting controlled amounts of air into the furnace to avoid non-oxidizing conditions therein, said non-oxidizing conditions tending-to retard the alloying between the iron and the zinc.

The subject material may also be produced by the process illustrateddiagrammatically in FIGURE 2. In this embodiment the galvanized strip is wound into a coil 23 following the coating step. A plurality of these coils 23 are then placed inside a conventionally constructed box-type annealing furnace 24 andheated until the desired amount of alloying has takenplace. As

The desired coating 1 I a temperature of 1200 F.

for

5 minutes, followed by cooling in air. Lower. alloying temperatures require longerperiods of time at the alloyabout 2 to 5 minutes, followedby cooling in air; or by, heating to about between 1000 F.fand ll- F. and holding forfa period of about.

ing temperature. For example, if the alloying tempera- 1 ture is below the melting'point of zinc (e.g., 750 F.) the time requiredato insure the necessary alloying may be several hourn p I Z 'It should be noted that there is no harm in exposing the galvanized material to alloying temperatures for. pe-

riods beyond the minimum period necessary to produce 7 the desired amount of alloying. In otherqwords, if the minimum time forproducing the desired amountof alloyingiat a low temperature such as 750? F. is 48 hours, 'no harm can be done by subjecting the. material to this alloying temperature for periods exceeding 48 hours. This is perhaps best illustrated by referring .to'curves vA and-B in the graph of FIGURE 4. Curve A illustrates the iron gradient curve for material subjected to an alloy throughout its thickness; More specifically, the coating of curve A has an Fe content varying from about .8% at the coating outer surface to 50% at the interface,'whereas the Fe content in the coating of curveB varies'from 20% at the outer surface to 50% at the interface.

.FIGURE 3 illustrates a third embodiment of a process which may be utilized to produce the subject material.

In this embodiment the, galvanized strip is initially heated in a continuous type annealing furnace, then wound into a coil 23, following which the coils23 are subjected to further alloying in a batch type box furnace .24. 1 A process utilizingtwo alloying steps eliminates the construction of expensive extensively lengthy continuous type annealing furnaces 20. This type of process can also be used to salvage galvanized material having dark gray coatings which are brittle. Brittle dark gray. coatings may be .the result of under-alloying during a process in.-

tended to produce the subject material; orrof over-alloy:

ing during a process intended to produce the light gray-, coated material described in. US. Letters Patent No.

3,05 6,694, referred to above,

Curve D in FIGURE 4 illustrates the iron content curve for a galvanized material having a brittle dark gray coating. By subjecting this material to a salvaging treatmentconstituting additionalalloying, the configuration of the curve canbe' changed from the convex type illustrated bycurve D to the concave type illustrated by curves. A

andB. The coating onthe salvaged galvanized material has the-same; desirable characteristics as. the coating on v material not requiring'the salvaging treatment.;

When coilsof galvanized material undergo alloying treatment by heating inbox-type furnaces, the coils may be tightly wound or they may be of the open'coil type with each of the'turns of the coil spacedfrom the others uniform. Tightly wound coils experience more alloying curve, and becauseevery turn of the coil would'have an iron content at. the outer surface of the coating greater than 7%. H r a There have thus been described typical processes for producing galvanized steel. material having .a smooth,

spangle-free, tightly adherent dark, gay coating ideally.

suited for painting. It should begunderstood that the subject invention is not limited to material produced by continuousvtype galvanizing processes. Batch-type processes orother processes well known to those skilledin the: art would be equally applicable, the important thing being that the galvanized material is subjected to an alloying step following the galvanizing step, which alloying step produces a coating having an iron content at the outer surface of the coating in excess of 7% and having an iron :gradient characterized by a concave type curve as illus- 14-24gaugewith about10.5 -l-.5 ouncesof zinc per.

7 "square foot; I

and then alloying said zincinsaid coating with iron from said steel material by heating said zinc-coated steel-material at an iron-zinc alloying temperature between 750 and 1900 F. fora timebetween seconds and hours. 1 2. A- process as recited in claimd wherein said'zinccoated steel mat-erial is heated atgan alloying temperature of about between 1800? F. .and'l900 F., w-hile said coating is still in'a substantially molten condition,- for around 60 seconds, v

3.. A process as recited in claim l'jwhe'rein' said zinccoated steel material is heated. at a temperature between 1000 -F. and 1900.F. fora timebetween 60 seconds and S-rninutes. I

4. A process as recited in claim '1 wherein said zinccoated steel material is heated at a temperature between coated steel material is heated immediate-lyrafter coating,

while the .coatinglis in a substantially molten condition, by .moving said zinc'acoat-ed; steel material througha con- I tinuous heating furnace. j

bythin' separators... Open coil heating is dvantageous in that it takes less timeand in that the alloying is'more in the outer turns than in the inner turns of the coil, but

' this need notbe'deleterious if alloying is continued until,

the inner turns have an iron gradient curve "such as that of curve A in'FIGURE 4. Under these circumstances the; outerturns of theicoillwouldhave a curve somewhat similar to curve B in FIGURE '4. Both inner and outer turns would have adherent, corrosion-resistant, daily-gray coatings,1because every turn of the coil would .havea coating with an iron'g radient characterized by a concave:

7 J. A processas reicited intclaim l wherein said alloyin'g step comprises:

moving. said zinc-coated .steelrnaterial through a con-1 tinuous heating furnace immediately. after saidlcoat-v ing step to. obtain a partial alloying; i .1 w x then coiling said material; Y I i p a and then; heating said coiled material having the partially' alloyed coating in a box furnace to complete, the'alloyin'gu. 1 1 8.7A process as recited inclaim'7whereinsaid coiled material .is in a tightly lcoiled condition-when-it is heated insaid box furnace. 7 i

V 9. A-process asrecited inclaim 1 and comprising;

coiling said zinc-coated (steel material after coating; and performing said "alloying jina box:furnace. I

amount: of the. desired a 5 6 10. A process as recited in claim 9 wherein said coil 2,719,820 10/55 Allen 20437 X is in a tightly coiled condition when said alloying is per- 2,986,808 6/61 S-chnedler. formed in said box furnace. 2,994,126 8/61 Kennedy 29196.5 3,056,694 10/62 Mehler 29196.5 X efe s Cited y the Examiner 5 3,078,555 2/63 McFarland 29196.5

UNITED STATES PATENTS OTHER REFERENCES 1,307,853 6/19 Dimm- I The Making, Shaping and Treating of Steel, 7th Edi- 1,468,905 9/23 Herman tion, 1957, pages 415 and 669-672. 1,944,227 9/31 Durbin 117-l14 X 1,948,505 2/34 Bray. 10 DAVID L. RECK, Primary Examiner. 5 23%? 352; HYLAND BIZOT, Examiner. 

1. A PROCESS FOR PRODUCING AN ADHERENT, CORROSIONRESISTANT, DARK GRAY GALVANIZED COATING ON FLAT STEEL MATERIAL, SAID PROCESS COMPRISING THE STEPS OF: COATING FLAT STEEL MATERIAL HAVING A THICKNESS OF ABOUT 14-24 GAUGE WITH ABOUT 0.5-1.5 OUNCES OF ZINC PER SQUARE FOOT; AND THEN ALLOYING SAID ZINC IN SAID COATING WITH IRON FROM SAID STEEL MATERIAL BY HEATING SAID ZINC-COATED STEEL MATERIAL AT AN IRON-ZINC ALLOYING TEMPERATURE BETWEEN 750*F. AND 1900*F. FOR A TIME BETWEEN 60 SECONDS AND 72 HOURS. 